It is important that the dwell time, which is to say the time for which the board is in contact with the solder, is within critical limits. It must be sufficient to ensure proper bonding but not so long as to cause any thermal damage to the components.
Clearly the conveying means is readily controlled. The solder wave is maintained by pumping molten solder over a weir for example. Changes in the pump operation can have a significant effect on dwell time.
Various instruments for analysing solder wave parameters are known, but such have failed to simulate the traditional technique of passing a sheet of glass over the wave and observing the area of contact between the wave and the underside of the glass.